The present invention relates to low density expanded thermally clad thermoplastic microspheres, preferably microspheres having a density of less than 0.015 g/cm3 and a process for making the same. The thermal cladding is a barrier coat which can be applied as a liquid, solid, or combination of liquids and solids.
Microspheres are heat expandable thermoplastic polymeric hollow spheres containing a thermally activatable expanding agent, such as those disclosed in U.S. Pat. No. 3,615,972, issued to Donald S. Morehouse. In the Morehouse patent, the microspheres are made in an aqueous system by the limited coalescence process under pressure, and the resulting product is a “wet-cake” of the unexpanded microsphere beads wetting agents and water. The wet-cake is typically about 40 to 75 weight percent solids, and because of the wetting agents employed in the formation of the beads, the surface will be wet. Many important uses of the microspheres require the removal of the water. Other teachings concerning such materials and uses are found in, for example, U.S. Pat. Nos. 3,864,181, 4,006,273, and 4,044,176.
It is common to expand the beads before use, such as, for example, in the production of syntactic foams in a wide variety of polymer matrices. In unexpanded form, the dry microsphere beads typically have a displacement density of about 1.1 g/cm3. Dry, expanded microspheres typically have a density by displacement generally in the range 0.06 g/cm3 to 0.02 g/cm3, and, preferably, are free-flowing. In the blending of these materials into a polymer binder system as a constituent in syntactic foam formulations, the exceptional ultra low density characteristic has led to drastic changes in the ways in which the weight and volume relationships of such materials are considered. As a general rule of thumb, the addition of one percent by weight of the expanded microspheres will represent the substitution of about twenty percent or more by volume of the host polymer in typical systems.
The characteristics of the microspheres have precluded many approaches to their drying and expansion. Severe agglomeration and adherence of the materials to warm surfaces of equipment have eliminated most approaches to such procedure from serious consideration. Wet expansion in steam is of limited use when an aqueous system is not wanted, and the spray drying procedure is so expensive and the product so prone to excessive, and extremely difficult dusting problems, along with substantial agglomeration and limited expansion of the beads.
The techniques in the prior art for the production of expanded microspheres produced from wet-cake are limited in at least one of two aspects: either the product remains wet, or the expanded beads are produced in dry form with substantial amounts of agglomeration and a limited degree of expansion; The “acceptable” levels of agglomeration achieved in such procedures are in the range of about 3 to 10% of the product.
Pre-expanded, dry, low density microspheres can produced by spray drying, as is taught by U.S. Pat. No. 4,397,799, but the spray drying procedure has several disadvantages, such as a dedicated spray drying facility is required, representing a very substantial capital investment, and very considerable operating expenses, particularly for skilled labor and utilities costs in heating the drying fluid, and the product is produced entrained in a heated, moving fluid stream, and the requirements of collection, recovery and handling are considerable. Experience has shown that the spray drying technique is suited for developing expanded densities no less than about 0.032 to about 0.040 g/cm3 and typically about 0.036 g/cm3. Attempts at lower densities result in unacceptable levels of agglomeration, greater than about 10% of the product, and equally unacceptable product losses attributable to over-expansion, with attendant disruption of the bead structure.
An acceptable process of making microspheres is disclosed in U.S. Pat. No. 4,722,943, the disclosure of which is incorporated by reference herein, which discloses a process wherein wet-cake mixed with a processing aid, dried and expanded in an integral operation. In the process, a processing aid is adhered to and embedded in the surface of the microspheres by thermal bonding, wherein the surface of the microspheres is heated to a temperature above the glass transition temperature, Tg, so that the polymer material behaved as a hot melt adhesive to bind the processing aid. The product is within the limits heretofore considered “acceptable”, with agglomeration levels in the range of 3 to 10%.
In U.S. Pat. No. 5,180,752, the disclosure of which is incorporated by reference herein, a technique for drying and expanding thermoplastic microspheres is disclosed, but that the teaching considers the expansion to be at the limit of expandability at a density of 0.015 g/cm3.
U.S. Pat. No. 5,580,656, the disclosure of which is incorporated by reference herein, relates to a further development in the drying and expansion operation, wherein the surface barrier coating is, in whole or in material part, a liquid material at drying temperatures. This disclosure also considers a density of 0.015 g/cm3 to be a lower limit on density.
It has been thought for some time in the art that the limits of expansion of thermoplastic microspheres had been reached at densities of about 0.015 g/cm3. Since the uses of such materials is most often directed to incorporation into formulations for the purpose of displacing mass, the lower the density of the microspheres, the more desirable and cost effective their use becomes. It has been common for those in the art to theorize that expansion of microspheres to lower densities was not possible because the wall thickness of the microspheres would be so reduced that the spherical hollow structure of the microsphere beads would shrink and “implode” as the blowing agent within the cellular structure cooled after expansion, or that the walls would be disrupted and break as such distention required for lower densities. Those of ordinary skill in the art have not made hollow thermoplastic microspheres at densities of less than 0.015 g/cm3, and have not supposed that such a feat is possible.